Male spontaneously hypertensive rats and Wistar-Kyoto controls age matched at 4, 6, 8 and 12 weeks were sacrificed and their thymocytes isolated. The sodium content and sodium efflux rate constants of the thymocytes were determined. Cell sodium was higher in SHRs. In both groups, total and ouabain sensitive efflux fell with increasing age. However, SHRs had lower values at each age studied for total, ouabain insensitive, and furosemide/insensitive efflux. Blood pressure and total peripheral resistance were higher in SHRs. These findings may be important in the development of hypertension in the SHR.